1. Field of the Invention
The present invention relates to position indicating systems generally and particularly to position indicating circuits for indicating the position of a nuclear reactor control rod.
2. Description of the Prior Art
Position indicating systems are generally known for indicating the position of a control rod in a nuclear reactor. Such known systems employ a permanent magnet mounted to the control rod to be movable internally of the reactor along with the control rod. A reed switch circuit is usually located externally of the control rod housing along the operating length of the control rod and hence along the operating length of the permanent magnet. As the control rod moves, different reed switches of the circuit are actuated by the magnetic flux of the permanent magnet and yield appropriate output signals whose magnitudes are indicative of the position of the control rod. These output signals are used to control the nuclear reactor.
The reed switches of such reed switch circuits have an inherent hysteresis characteristic. Namely, the reed switch requires a high magnetic flux level .phi..sub.1 to close the reed switch and once closed the reed switch will remain closed until the magnetic flux level drops down to low magnetic flux level .phi..sub.2 significantly lower than .phi..sub.1. This magnetic flux level difference results in an uncertainty as to the actual position of the control rod since the permanent magnet must be in a first position close to the reed switch to actuate the reed switch to close and must be significantly backed away from the reed switch to open the reed switch. Thus a signal from a closed reed switch can only indicate that the control rod is somewhere between the first and second control rod positions with respect to the reed switch. This factor prevents the use of reed switches in more accurate nuclear reactor controls.